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Invasive Species

"Invasive species"—it may not sound very threatening, but these invaders, large and small, have devastating effects on wildlife. Invasive species are among the leading threats to native wildlife. Approximately 42 percent of threatened or endangered species are at risk due to invasive species.

Human health and economies are also at risk from invasive species. The impacts of invasive species on our natural ecosystems and economy cost billions of dollars each year. Many of our commercial, agricultural, and recreational activities depend on healthy native ecosystems.

What makes a species invasive?

An invasive species can be any kind of living organism—an amphibian (like the cane toad), plant, insect, fish, fungus, bacteria, or even an organism’s seeds or eggs—that is not native to an ecosystem and causes harm. They can harm the environment, the economy, or even human health. Species that grow and reproduce quickly, and spread aggressively, with potential to cause harm, are given the label “invasive.”

An invasive species does not have to come from another country. For example, lake trout are native to the Great Lakes, but are considered to be an invasive species in Yellowstone Lake in Wyoming because they compete with native cutthroat trout for habitat.

How Invasive Species Spread

Invasive species are primarily spread by human activities, often unintentionally. People, and the goods we use, travel around the world very quickly, and they often carry uninvited species with them. Ships can carry aquatic organisms in their ballast water. Insects can get into wood, shipping palettes, and crates that are shipped around the world. Some ornamental plants can escape into the wild and become invasive. And some invasive species are intentionally or accidentally released pets. For example, Burmese pythons are becoming a big problem in the Everglades.

In addition, higher average temperatures and changes in rain and snow patterns caused by climate change will enable some invasive plant species—such as garlic mustard, kudzu, and purple loosestrife—to move into new areas. Insect pest infestations will be more severe as pests such as mountain pine beetle are able to take advantage of drought-weakened plants.

Threats Posed by Invasive Species

Invasive species cause harm to wildlife in many ways. When a new and aggressive species is introduced into an ecosystem, it might not have any natural predators or controls. It can breed and spread quickly, taking over an area. Native wildlife may not have evolved defenses against the invader or they cannot compete with a species that has no predators.

The direct threats of invasive species include preying on native species, outcompeting native species for food or other resources, causing or carrying disease, and preventing native species from reproducing or killing a native species' young.

There are indirect threats of invasive species as well. Invasive species can change the food web in an ecosystem by destroying or replacing native food sources. The invasive species may provide little to no food value for wildlife. Invasive species can also alter the abundance or diversity of species that are important habitat for native wildlife. Aggressive plant species like kudzu can quickly replace a diverse ecosystem with a monoculture of just kudzu. Additionally, some invasive species are capable of changing the conditions in an ecosystem, such as changing soil chemistry or the intensity of wildfires.

Examples of Invasive Species

Asian Carp: Asian carp are fast-growing, aggressive, and adaptable fish that are outcompeting native fish species for food and habitat in much of the mid-section of the United States. The huge, hard-headed silver carp also pose a threat to boaters, as the fish can leap out of the water when startled by boat engines, often colliding with people and causing injuries. "Asian carp" is a catchall name for species of silver, bighead, grass, and black carp from Southeast Asia. Voracious filter feeders, Asian carp consume up to 20 percent of their body weight each day in plankton and can grow to more than 100 pounds.

Asian carp were imported to the United States in the 1970s to filter pond water in fish farms in Arkansas and quickly spread across the country. Flooding allowed them to escape and establish reproducing populations in the wild by the early 1980s. Asian carp are swiftly spreading northward up the Illinois River, and are now on the verge of invading the Great Lakes. Once established in an ecosystem they are virtually impossible to eradicate. Adult Asian carp have no natural predators in North America and females lay approximately half a million eggs each time they spawn.

Temperatures in the Great Lakes are well within the fishes’ native climate range. Parts of the Great Lakes, including nutrient-rich bays, tributaries, and other near-shore areas, would offer Asian carp an abundant supply of their preferred food, plankton. Plankton is also favored by most young and many adult native fishes and the voracious carp would likely strip the food web of this fundamental resource. The U.S. Geological Survey has identified 22 rivers in the U.S. portion of the Great Lakes that would provide suitable spawning habitat for Asian carp.

Brown Marmorated Stink Bug: The brown marmorated stink bug, Halyomorpha halys, is native to China, Japan, and surrounding countries. They were first discovered in the United States in Pennsylvania during the late 1990s, but no one knows for certain how they were introduced to North America. Brown marmorated stink bug (BMSB) populations are exploding in the absence of their natural predators, and they are quickly becoming a nuisance to people in their homes and to the agriculture industry.

A big problem with BMSBs so far is the infestation of people’s homes. The bugs begin to come indoors, searching for warm, protected areas when outside temperatures turn cooler in the fall. They don’t reproduce inside the home or cause structural damage, but their namesake odor, noisy flying, and teeming numbers can make the BMSB an extreme nuisance throughout the winter, especially on warmer days when they are more active.

BMSBs feed on host plants by piercing the skin and consuming the juices within; the signs of stink bug feeding appear as "necrotic" or dead spots on the surface. They've become a significant agricultural pest in the mid-Atlantic region, and other areas could see similar effects if the BMSB’s range continues to expand. A wide variety of plants are known food sources for BMSBs, including ornamental trees and shrubs; fruit crops like peaches, apples, grapes, and pears; vegetable crops like green beans and asparagus; and soybeans and corn.

Zebra Mussels: Zebra mussels and quagga mussels are virtually identical, both physically and behaviorally. Originally from Eastern Europe, these tiny trespassers were picked up in the ballast water of ocean-going ships and brought to the Great Lakes in the 1980s. They spread dramatically, outcompeting native species for food and habitat, and by 1990, zebra mussels and quagga mussels had infested all of the Great Lakes. Now both quagga mussels and zebra mussels have spread to 29 states by hitching rides on boats moving between the Great Lakes and Mississippi River Basins. Artificial channels like the Chicago Area Waterways System facilitate their spread. These man-made channels act like super-highways and are also a pathway for Asian carp, which are currently spreading towards the Great Lakes.

The quagga and zebra mussels blanketing the bottom of the Great Lakes filter water as they eat plankton and have succeeded in doubling water clarity during the past decade. Clear water may look nice to us, but the lack of plankton floating in the water means less food for native fish. Clearer water also allows sunlight to penetrate to the lake bottom, creating ideal conditions for algae to grow. In this way, zebra and quagga mussels have promoted the growth and spread of deadly algae blooms.

Zebra and quagga mussels harm native fish populations, ruin beaches and attach to boats, water intake pipes, and other structures, causing the Great Lakes economy billions of dollars a year in damage. They devastate native species by stripping the food web of plankton, which has a cascading effect throughout the ecosystem. Lack of food has caused populations of alewives, salmon, whitefish, and native mussel species to plummet.

In her five-year lifetime, a single quagga or zebra mussel will produce about five million eggs, 100,000 of which reach adulthood. The offspring of a single mussel will in turn produce a total of half a billion adult offspring. There are an estimated 10 trillion quagga and zebra mussels in the Great Lakes today. Once zebra and quagga mussels become established in a water body, they are impossible to fully eradicate. Scientists have not yet found solutions that kills zebra and quagga mussels without also harming other wildlife.

Additional examples of invasive species:

Curbing the Spread of Invasive Species

One way to curb the spread of invasive species is to plant native plants and remove any invasive plants in your garden. There are many good native plant alternatives to common exotic ornamental plants. In addition, learn to identify invasive species in your area, and report any sightings to your county extension agent or local land manager.

Regularly clean your boots, gear, boat, tires, and any other equipment you use outdoors to remove insects and plant parts that may spread invasive species to new places. When camping, buy firewood near your campsite (within 30 miles) instead of bringing your own from home, and leave any extra for the next campers. Invertebrates and plants can easily hitch a ride on firewood you haul to or from a campsite—you could inadvertently introduce an invasive to a new area.


PennState College of Agricultural Sciences
The Ohio State University
University of Maryland
U.S. Department of Agriculture

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